This invention relates to a device for micropositioning of objects, for instance in microscopy. The device comprises an accelerating means and a positioning unit connected with the accelerating means and in turn connected with the object. Moreover the position of the object relative to the positioning unit is variable at high acceleration or retardation of the accelerating means, owing to mechanical inertia of the object.
In many fields there is a need for positioning of objects with great accuracy. This is most important, for example in a scanning probe microscope, SPM, in which a sample must be moved towards the probe before starting a scanning. A technique of carrying out such moving involves an inertia-type motor, according to the above description, in which excitation of a piezotube causes an extension of the same and a corresponding movement of an object connected therewith, whereupon the piezotube is very rapidly retracted as the excitation ceases, and owing to moment of inertia in the system, the object that is to be moved then stays in the position where it was located when the piezotube was in its excited position. A device of the above type is described in, for example, K. Svensson, F. Althoof and H. Olin, xe2x80x9cA compact inertial slider STMxe2x80x9d, Meas. Sci. Techn., 8, 1360-1362 (1997). This publication describes a device for micropositioning in a scanning tunnel microscope, comprising both a sample and a sharp scanning tip. The position of the sharp scanning tip relative to the sample is controlled by means of two concentric piezoelectric tubes, the inner tube being used for scanning of the prod and the outer tube being used for inertia movement of the sample. The construction also comprises a positioning unit including two parts, a first part which is fixedly connected to the outer tube and a second part which holds the sample. The second part is slidingly arranged on the first part, the sliding surface being located in a plane which is not perpendicular to the sharp scanning tip. When applying, for example, a serrated waveshape to the piezoelectric element, displacements between these two parts occur as the acceleration exceeds the limit of the static friction between the parts. The two parts are arranged in such manner that, when sliding towards each other in said sliding surface, the sample arranged on the second part is moved so as to be closer or further away from the prod while at the same time the actual sample surface is continuously held perpendicular to the prod. This construction is very compact and has a number of desirable properties, such as a low noise level. However the construction is complicated and comprises two piezoelectric elements. A micro-positioning device of a simpler design is therefore desirable, which has a short mechanical loop, to reduce the mechanical noise in the form of vibrations in the system. A simpler design of the system further contributes to reducing the risk that dirt and other external interference reduces or fully eliminates the function of the inertia-type motor. Furthermore the above prior-art construction is particularly adapted to scanning tunnel microscopy (STM), and a more general device for use in connection with e.g. scanning probe microscopy (SPM) and other applications is desirable.
The object of the invention thus is to provide a micropositioning device which is of a simple design and which is adaptable for use within a plurality of applications, such as in scanning probe microscopy.
According to the invention this and other objects are achieved by the positioning unit, included in the device, comprising at least two clamping elements, between which the object is adapted to be held merely by means of the clamping force and the frictional force exerted by said clamping elements. Consequently, the object is movable relative to the clamping elements by mechanical inertia when the acceleration/retardation force obtained for the clamping elements from the accelerating means connected thereto exceeds said clamping and frictional forces between the object and the clamping element.
The clamping elements conveniently comprise resilient tongues which are directly connected with the accelerating means and are oriented in the longitudinal direction of the accelerating means and between which the object is arranged. This allows a simple construction and easy attachment of the object between the resilient tongues.
According to a preferred embodiment, said object is spherical, which, in cooperation with suitably arranged accelerating means, by means of separate acceleration/retardation control of each clamping element, not only allows movement of the object away from and towards the accelerating means (in the z direction), but also rotation of the spherical object in two directions (up and down, as well as laterally relative to the clamping elements).
According to another embodiment, the object is cylindrical, which allows increased mechanical stability. The term xe2x80x9ccylinderxe2x80x9d is here to be interpreted in its mathematical definition, i.e. a body defined by a surface which is obtained by parallel displacement of a straight line along a closed curve.
According to an embodiment, the longitudinal direction of said cylindrical object is essentially parallel to the longitudinal direction of the accelerating means, i.e. the cylinder is moved in its longitudinal direction when moving the object away from or towards the accelerating means. This causes a stable motion in the z direction owing to a comparatively large abutment surface between the cylinder and the clamping elements.
Suitably said cylinders are essentially circular in cross-section, which allows simple, angle-independent positioning of the cylinder between the clamping elements.
According to one more alternative embodiment, said positioning unit comprises two U-shaped means with legs of a sheet-like material, which are joined in such manner that their respective open ends are oriented in opposite directions, the legs of the first U-shaped means constituting said clamping element for forming a sliding surface against said object, and the legs of the second U-shaped means constituting sliding surfaces intended for direct abutment and high-friction-sliding against the accelerating means. This construction allows easy exchange of the positioning unit, for example in case of wear on the resilient elements or the sliding surfaces. Moreover, if necessary, various positioning units can be used with the same piezoelectric element. The object in this embodiment is suitably cylindrical and circular in cross-section and its longitudinal axis is essentially perpendicular to the longitudinal direction of the accelerating means. This allows stable attachment in the longitudinal direction of the object, between the resilient elements of the first U-shaped means and the circumferential surface of the cylindrical object, and also in the z direction, between the resilient elements of the second U-shaped means and the circumferential surface of the piezotube, in both cases owing to a larger abutment surface than in the spherical case. Said sliding surfaces preferably comprise sliding elements, preferably graphite rods, which are arranged on the leg surface and directed towards the object and the accelerating means, respectively. These rods allow, in addition to excellent sliding quality, also guiding of the object in the desired direction.
Preferably the accelerating means further comprises a tube of piezoelectric material, which provides a simple and well-tested mechanism for obtaining said rapid acceleration and retardation motions.
According to one more preferred embodiment, the clamping elements are arranged inside the tube of piezoelectric material, which allows a very compact and space-efficient construction.
Finally a sample holder is suitably arranged on the object, on the side which is essentially directed away from the accelerating means. This allows the sample holder to be formed separately from the object and thus to have a shape suitable for the sample, independently of the shape of the object. This allows on the other hand that the object can be designed for ideal possibilities of adjustment.